Pharmacokinetic studies determined a plasma concentration of arecoline, a cholinergic agonist that improved memory in Alzheimer's disease (AD) patients and predicted an optimal dose in all subjects. Arecoline administration improved different cognitive functions at different doses. Verbal ability improved at low doses, whereas attention and visuospatial ability improved at higher doses. Improvement in cognition was not due to activation of the hypothalamic-pituitary-adrenal axis. Administration of physostigmine, an inhibitor of brain acetylcholinesterase, also showed a modest improvement in verbal memory in most AD patients. Memory enhancement was correlated with plasma butyrylcholinesterase inhibition but not with plasma physostigmine concentration. Based on reported reductions of in the concentrations of the cofactor tetrahydrobiopterin and catecholaminergic neurotransmitters in cerebrospinal fluid of AD patients, a therapeutic trial using tetrahydrobiopterin was initiated in AD patients.